Glossary of Watershed Terms

Download the Glossary from the SLO County Watershed Management Plan. Click here.
Active Channel The channel that contains the discharge where channel maintenance is most effective, sediment are actively transported and deposited, and that are capable of containing most flows. Active channels are located within the area bounded by bankfull stages.
Active Flood plain Low lying areas built by watercourse sediment depositions between top of bank that are adjacent to a watercourse and that have been constructed by the present river in the present climate. These areas are susceptible to frequent inundation during moderate and higher flows when the active channel’s capacity is exceeded. Active floodplains are most prominent along low-gradient, meandering reaches and are often absent or undistinguishable along steeper sloped stream channels.
Anadramous Fish Populations

Anadromous fish are born in fresh water, and then migrate to the ocean to grow into adults, and finally return to freshwater to spawn. Salmonid species (e.g. salmon, steelhead trout) are anadramous fish.

Bankfull stage Bankfull stage is the point at which the flow just begins to enter the active floodplain. Accurate measurements have been conducted on gaged streams, however, in absence of historical hydrological records there are a number of field indicators that can be used to identify bankfull stages with a great deal of accuracy: • An abrupt change in the slope of the stream channel, usually from a vertical plane to a horizontal plane on top of the floodplain. • The bankfull stage is usually marked by a change in vegetation such as the change from gravel bars to forbs, herbs, or grasses. Persistent woody vegetation is usually indicative of upland terrain, but can be misleading. • Erosion or scour features. These features indicate areas just below the bankfull stage and are recognized as significant characteristics of stream dynamics. • Flat depositional benches, lateral bars, or point bars, usually created by lateral or downward movement of streams and can create active floodplain areas. • Change in the size distribution of sediment materials at the surface from fine gravel to cobbles, from sand to gravel or even fine gravel material. It can change from fine to coarse or coarse to fine. • Stain lines can indicate frequent inundation of water on rocks. Stain lines may be marked by sediment or lichens.

The engineered straightening and/or lining of natural, meandering creekbeds using rock (riprap) or concrete. Traditionally implemented for flood-control purposes; has resulted in some of the most detrimental flood control projects that exist, including exacerbating erosion and flooding problems in downstream areas, and eliminating critical ecological habitat (such as fish habitat and vegetative cover on stream banks). The loss of natural stream meander increases flooding problems, while increasing water flow speeds and volumes.

Geomorphology (Fluvial) The study of the natural relationship between a stream and its bank and bed. Forced into an unnatural position, the streambanks and bed will erode. Shaped to match the water flow, the stream remains more stable.
Habitat The specific area or environment in which a particular type of plant or animal lives. To be complete, an organism's habitat must provide all of the basic requirements of life for that organism.
Hydrology 1. The branch of physical geography concerned with the behavior of water in the atmosphere, on the surface of the earth and underground. 2. The science dealing with the properties, distribution and circulation of water
Infill The development of the last remaining lots in an existing developed area, the new development within an area already served by existing infrastructure and service, or the reuse of already developed, but vacant properties.
Outside Edge of Riparian Habitat The riparian edge is the outer boundary of the existing riparian vegetation; for trees, the dripline is the outer boundary.
Restoration The reestablishment of the structure and function of ecosystems. Ecological restoration is the process of returning an ecosystem as closely as possible to predisturbance conditions and functions. Implicit in this definition is that ecosystems are naturally dynamic. It is therefore not possible to recreate a system exactly. The restoration process reestablishes the general structure, function, and dynamic but self-sustaining behavior of the ecosystem.
Riparian 1. On, or pertaining to, the banks of a stream. (As in riparian vegetation or riparian woodland.) 2. Pertaining to the banks and other adjacent, terrestrial (as opposed to aquatic) environs of freshwater bodies, watercourses, and surface-emergent aquifers (e.g., springs, seeps, oases), whose imported waters provide soil moisture significantly in excess of that otherwise available through local precipitation -soil moisture to potentially support a mesic vegetation distinguishable from that of the adjacent more xeric upland.
Riparian Buffer Land next to a stream or river that is vegetated, usually with trees and shrubs, that serves as a protective filter for streams. A buffer helps to stabilize stream banks from washing away and to reduce the impact of upland sources of pollution by trapping, filtering, and converting sediments, nutrients, and other chemicals. In addition, a buffer helps supply food, cover, and thermal protection to fish and other wildlife. Riparian buffers can be 300 feet wide or 20 feet wide; it depends on the stream and the land around the stream.
Riparian Vegetation 1. Vegetation which occurs in and/or adjacent to a watercourse. For the purpose of administering Fish and Game Code Section 1600, et seq., this should be expanded to include vegetation adjacent to lakes as well. 2. Vegetation growing on or near the banks of a stream or other body of water on soils that exhibit some wetness characteristics during some portion of the growing season. 3. Vegetation which occurs along watercourses, and is structurally or floristically distinct from nearby, non-streamside vegetation.4. Riparian vegetation is terrestrial vegetation that grows beside rivers, streams, and other freshwater bodies and that depends on these water sources for soil moisture greater than would otherwise be available from local precipitation.

Heavy stones used to protect soil from the action of fast-moving water. True restoration tries to eliminate or minimize the use of riprap, using plants with strong root systems to anchor soil instead (e.g. willows). The use of riprap can lead to many detrimental effects, including increasing water volumes and flow (which leads to flooding and bank erosion), and constricting stream channels (which exacerbates flooding).


The use of natural methods to control creek bank erosion, and to restore natural stream habitat. Also referred to as, "biotechnical slope protection," soil-bioengineering involves the use of live and dead woody cuttings and poles or posts collected from native plants, to revegetate watershed slopes and stream banks. The cuttings, posts, and vegetative systems composed of bundles, layers, and mats of the cuttings and posts provide structure, drains, and vegetative cover to repair eroding and slumping slopes.

Spawning and Spawning Habitat

The method by which salmonids reproduce and deposit their eggs. Salmonids, such as Steelhead trout, return to the freshwater streams of their birth, to lay their eggs. The prime spawning habitat is in the small, upstream waterways (the "headwaters" and "tributaries" of larger rivers), where the water temperature is cooler, and there are gravel beds in the stream channel. The female uses her body to dig a depression in the gravel, and lays her eggs there, while a male waits at the side to fertilize the eggs. The female then moves upstream and flaps her tail against the stream bottom, dislodging gravel which covers the egg-laying site. The eggs incubate in this nest, until they emerge as "fry”.

Stream/Channel/Creek A stream is a body of water that flows at least periodically or intermittently through a bed or channel having banks. This may include watercourses having a surface or subsurface flow that supports or has supported riparian vegetation, fish and/or aquatic life.
Stream bed The substrate plane bounded by the stream banks over which water moves. Also called stream bottom. It is the area kept mostly or completely bare of vegetation by the wash of water of the stream.
Stream bank The portion of the channel cross section that restricts lateral movement of water. A distinct break in slope from the channel bottom.

The landscape area whose run-off flows into a particular stream or river, also known as "basins" or "drainages." Watersheds can be nested within each other; smaller watersheds draining into smaller streams are nested within a larger encompassing watershed that contains all of those streams, as well as a larger river into which they flow.


Adapted from